1. Field of the Invention
This invention relates to modem communications. More particularly, the invention pertains to a system and method for supporting high speed, bi-directional analog modem communications at a data rate of substantially 64 Kbps. Still more particularly, the invention concerns a line card providing an interface between an analog modem and a digital transmission network, such as the digital portion of the Public Switched Telephone Network (PSTN).
2. Description of the Prior Art
Considerable attention has been directed toward increasing the data transfer rates of analog modems communicating over conventional local loop POTS (Plain Old Telephone Service) lines. In accordance with the V.90 standard currently in use, the maximum data rate for analog modems operating in the downlink direction (server to client) is 56 Kbps. In the up-link direction (client to server), the current maximum data rate is 33.6 Kbps.
Several factors limit analog modem technology to these speeds. First, filtering implemented at the line card interface between the local loop and the digital portion of the PSTN restricts available bandwidth to approximately 3000 Hz. Second, FCC regulations impose a maximum power limit of -9 dBm for analog signals carried on POTS lines terminating at RJ-11 jacks. This power limit reduces signal to noise ratios and also impairs the detection and processing of high frequency signal components, which are attenuated by filter-induced roll-off. Third, the analog-to-digital conversion and i-law or A-law companding performed by the line card CODEC (enCODer/DECoder), which is intended to reduce quantization noise, limits the CODEC's ability to use more than seven bits of each eight bit PCM codeword for the digital-to-analog conversion performed in the downlink direction. Conventional companding, moreover, has failed to increase the quantization error signal-to-noise floor above approximately 38-42 dB.
Accordingly, there is a need in the modem communication art for increased analog modem speed, in both the up-link and downlink directions. What is required is a system and method that improves signal processing efficiency at the interface between an analog modem and a digital transmission network to which it is connected in order to provide high speed, bi-directionally symmetrical modem communication. In a digital telephone system implementing 64 Kbps PCM codeword transmission, an improved line card at the local switch is indicated, such that bi-directional 64 Kbps analog modem communication may be facilitated on the local loop.